Because the official UMTS standard used in Europe is finding increasing acceptance, there is a corresponding increase in both the need for further frequency bands and the complexity of mobile phones. While the market deals with mobile phones which are able to work in one UMTS band (band 1) and in four GSM bands, the manufacturers are developing mobile phones and data cards adapted to work in three or four WCDMA bands and, at the same time, support GSM/GPRS and EDGE.
Thus, such a mobile phone needs up to six separate power amplifiers for transmitting in these seven or eight different frequency bands. There is also a need for a higher integration of these power amplifiers into one single module in order to save space. At the same time, costs can be reduced, which will be greatly advantageous to manufacturers of mobile phones.
There is a principal difference between power amplifiers which can be used for GSM/GPRS and power amplifiers which can be used for WCDMA (because of their basically different design). As such, in some examples, similar power amplifiers are integrated into one module working in a number of different frequency bands. The fundamental difference between an amplifier for GSM and an amplifier for WCDMA is relates to the different power level for TX signals. The maximum power levels of the amplifiers differ by more than 3 dB. While power amplifiers used for GSM/GPRS work with compression and high efficiency, power amplifiers for EDGE and WCDMA need to amplify signals linearly because the form of these signals has a strong amplitude component. There are also differences in the respective matching to the different modes of amplifying. The impedance of the harmonic waves needs to be very low (harmonic short). In GSM the uneven harmonic waves need to be terminated with high impedance to increase efficiency.
There are already products where two similar power amplifiers used for a multitude of frequency bands are combined in a common housing. In a known product one amplifier each is used to amplify to frequency bands which are adjacent to each other. For example, one power amplifier for GSM 850+GSM 900 is combined with another power amplifier for GSM 1800 and GSM 1900 in a common housing. By this way, a single driving circuit can be used to drive these two power amplifiers which will save costs.
The same kind of integration is used in products where linear power amplifiers for WCDMA signals are combined in a single housing. But a problem is that it is nearly impossible to combine different functional circuits such that they can be used for the different amplifiers.
Another problem arises with linear power amplifiers while working at a reduced power level at the output. Working at a power level backed off from the maximum power level always leads to a reduction of the efficiency of the power amplifier. Thus, waste of energy and waste of battery capacity arise, resulting in reduced handset autonomy. To circumvent these problems, circuits and concepts have been developed for power amplifiers having two or more amplifier stages. There are working-modes where one or more of these power amplifier stages are by-passed or simply switched off. All these alterations, however, have the specific disadvantage of fundamental power losses in amplifier modes of reduced power level. These losses are based on the high capacity of the amplifier stage being recently higher than necessary on the reduced power level mode. Further, a complicated matching is necessary which produces further losses of energy in the circuit. Additional losses in a passive or a switched bypass matching network result from the necessary high impedance transformation ratio, e.g. from 4 ohm to 30 ohm. Further, if load impedance in a low power mode is not optimal additional efficiency reduction arises due to backed-off operation. In a switched bypass, resistive losses of the switch arise.
Typical coefficients of efficiency resulting at those amplifiers are about 40% when working at the highest power level, but reach at maximum a coefficient of 25% while working in medium power mode. This is regarded to be inefficient.
All known integrated amplifier products using the above mentioned bypass structures are working in a narrow band because of a cascaded matching at the power output. Therefore, it is impossible to simultaneously reach a broad-band matching if trying to save energy while bypassing one or more of a multitude of power amplifier stages.